1. Field of the Invention
The present invention relates to a multilayer wiring substrate and a method of producing the multilayer wiring substrate which is applicable to a package substrate incorporating electronic parts. More specifically, the present invention relates to the method of producing the multilayer wiring substrate which is free of a core substrate and has a metal supporting frame body formed on one of main surfaces of a build up layer.
2. Description of the Related Art
Recently, speed of digital product is more and more increased remarkably. According to the above high speed trend, the package substrate used for the digital product is getting smaller and having the increased number of connecter terminals (pins), so as to meet high integration as well as high density of electronic parts including LSI (=Large Scale Integration), IC (=Integrated Circuit) chip and chip capacitor which are mounted on the package substrate. This signifies the need for high wiring density of the package substrate.
For producing the package substrate with high wiring density, a build up layer method attracts attention recently. With layer-to-layer insulator films made of resin material, the build up layer method forms a laminated insulator layer on a core substrate. Then, the thus laminated insulator layer is overlapped with a wiring layer, thus forming a multilayer constitution so as to finally form the build up layer.
Japanese Patent Unexamined Publication No. Heisei 11 (1999)-233937 (=JP11-233937) and Japanese Patent Unexamined Publication No. Heisei 11 (1999)-289025 (=JP11-289025) disclose a wiring substrate having a build up layer on a core substrate.
Although bringing about the high density wiring, the build up layer formed through the above build up layer method can no longer keep pace with the demanded speed of the above digital product. This is attributable to the core substrate which is a member for compensating the build up layer for mechanical strength.
More specifically described as below:
Case 1: Provided that the core substrate is mounted in the package substrate, a space (or area) occupied by forming the core substrate is so designed as to be useless for the high density wiring of the package substrate. This uselessness may hinder still higher density wiring of the package substrate.
Case 2. The electric signal for the increased speed of the digital product may become as high as some G (giga) Hz in frequency. With the high frequency of the electric signal, the wiring connecting the drive source (of the electronic parts mounted in the package substrate) and the electronic parts may contribute as an inductance, thus rendering the wiring length considerable. In other words, thickness of the core substrate is not ignorable.
Without formation of the core substrate, a new type of package substrate has been proposed having the feature of the build up layer which enables high density wiring.
FIG. 5 shows the new type of package substrate having the above feature. There is formed a build up layer 80 including a wiring layer 110 and an insulator layer 90. A core substrate for compensating the build up layer 80 for mechanical strength is not shown in FIG. 5. Replacing the core substrate, a metal supporting frame body 70 formed on one (upper in FIG. 5) of main surfaces of the build up layer 80 compensates the build up layer 80 for the mechanical strength. The main surface (lower in FIG. 5) of the build up layer 80 is formed with a first metal pad layer 40, while the main surface (upper in FIG. 5) of the build up layer 80 is formed with a second metal pad layer 65. Each of the first metal pad layer 40 and the second metal pad layer 65 included in the build up layer 80 has an exposed surface. Hereinabove, the second metal pad layer 65 on the main surface (upper in FIG. 5) of the build up layer 80 becomes a connector terminal for mounting the electronic parts. On the other hand, the first metal pad layer 40 on the main surface (lower in FIG. 5) of the build up layer 80 becomes a connector terminal to be mounted, for example, on a mother board.
Hereinafter in the specification of the present invention, the package substrate having no core substrate and having the build up layer as a multilayer wiring layer is referred to as “multilayer wiring substrate.”
Japanese Patent Unexamined Publication No. 2002-26171 (=JP2002-026171) discloses a multilayer wiring substrate like the one in FIG. 5.
Adopting the multilayer wiring substrate for the package substrate (see typical drawing in FIG. 5) can bring about still higher density wiring, although leaving the following inconveniences.
The metal supporting frame body in FIG. 5 is so formed as to compensate the build up layer for the mechanical strength. Unlike the core substrate according to Japanese Patent Unexamined Publication No. Heisei 11 (1999)-233937 (=JP11-233937) and Japanese Patent Unexamined Publication No. Heisei 11 (1999)-289025 (=JP11-289025), the metal supporting frame body in FIG. 5 does not coat an entire region of the build up layer. Thereby, applying an external force is likely to cause concentrated stress to an area in the vicinity of an interface between the wiring layer and the insulator layer of the build up layer.
Referring to the typical drawing in FIG. 5, the second metal pad layer 65 with the exposed surface has such a small contact area with the insulator layer 90 as to cause a failure (including cracks and the like) to the second metal pad layer 65 when concentrated stress is applied. In addition, applying the concentrated stress in production steps to the area in the vicinity of the interface between the wiring layer and the insulator layer of the build up layer may also cause the cracks and the like.
As described above, the failure including the cracks and the like caused to the area in the vicinity of the interface between the wiring layer and the insulator layer of the build up layer may be responsible for lowered quality of the build up layer, including electrical characteristic and so on. The failure becoming great may cause defective product.